1. Field of the Invention
The present invention relates generally to encapsulating materials and methods for encapsulating photovoltaic devices. In particular, this invention relates to a composition of an EVA encapsulant which minimizes discoloration, and more particularly, to an encapsulating material having enhanced photothermal and photochemical stability, and reduced concentrations of curing-generated chromophores and curing peroxide residue, and to photovoltaic devices comprising these materials.
2. Description of the Prior Art
Solar energy conversion systems experience a unique set of stresses which may affect their stability and, hence, their overall performance and efficiency in converting the dilute radiant flux into electrical power. These stresses include ultraviolet (UV) radiation, extreme temperature fluctuations, atmospheric gases and pollutants, the diurnal and annual thermal cycles, and, in concentrating systems, a high-intensity solar flux. In addition to these detrimental environmental elements, a variety of other factors, such as rain, hail, dust, wind, thermal expansion mismatches, and condensation and evaporation of water, may further reduce the performance of the solar system. To maintain long-term stability and performance, the photovoltaic (PV) module must be sealed within a suitable protective encapsulant.
The encapsulating material (commonly referred to as "pottant"), which provides physical isolation and protection for the solar cell assembly, is a critical component of the PV module. In addition to protecting the semiconducting material(s) from intense environmental impact, the pottant also provides structural support, optical coupling, electrical isolation, and thermal conduction for the arrays of solar cells and current-collecting metallic components within the PV module. See Cuddihy, E. F., et al., Polymers in Solar Energy Utilization, C. G. Gebelein, D. J. Williams and R. D. Deanin, eds. (ACS, Washington, D.C., 1983), pp. 353-366; Lewis, K. J., Polymers in Solar Energy Utilization, ibid., pp. 367-385; Rabel, J. F., New Trends in the Photochemistry of Polymers, N. S. Allen and J. F. Rabek, eds. (Elsevier, Amsterdam, 1985), pp.265-288; and Cuddihy, E.F., et al., Flat-Plate Solar Array Project Final Report, Vol. VII: Module Encapsulation, JPL Publn. 86-11 (1986). Thus, to achieve a stable power output with a desired service life of over 20-30 years, both the encapsulant and the solar cell components must exhibit long-term weathering stability and reliable performance.
To be useful as an encapsulant for crystalline silicone (c-Si) solar cells, the encapsulating material must comprise a transparent polymer, preferably ethylene vinyl acetate (EVA), which affords good optical transmission in a prescribed spectral region, i.e., between about 290 nm and 2500 nm, and more specifically from about 380 nm to about 1200 nm. However, like other polyolefins, EVA copolymers tend to discolor as a result of thermal, photochemical, and/or photothermal degradation. Photothermal degradation develops, for example, when impurities within the polymeric matrix, such as UV-excitable chromophores, trace metals and other ionic species, function as "activation sites" for free radical formation. These activation sites are actuated by short wavelength UV light (below 385 nm) which, in the presence of oxygen, convert hydroperoxides and peroxides into free radicals. The UV-excitable chromophores are critical factors in the initiation of photo-oxidative reactions which generate hydroperoxides and thus free radicals. These free radicals, directly or indirectly via subsequent degradation reactions, induce cross-linking and/or chain scission reactions, both of which degrade the mechanical properties of the encapsulant. In particular, chain scission reactions sever intramolecular bonds, which reduces the molecular weight of the polymer and affects mechanical properties such as elongation to break. Cross-linking reactions, on the other hand, increase covalent bond formation between polymer chains, which reduces the elasticity and tensile strength of the polymer material. In addition to degrading the structural integrity of the polymer matrix, both chain scission and cross-linking reactions ultimately affect the permeability of the encapsulant, which further reduces its protective value.
The present inventor has determined that UV irradiation from exposure to sunlight causes significantly more damage to PV-module encapsulating materials than thermal degradation. To protect against UV-induced degradation, and thus improve weathering stability, conventional encapsulating materials include using stabilizing additives such as UV absorbers (UVA), UV light stabilizers (UVS), and antioxidants. The primary function of UVA is to absorb the damaging UV light and dissipate the light energy into heat or re-emit the energy as harmless light of longer wavelengths. A common UVA material is 2-hydroxy-4-n-octyloxy-benzophenone, manufactured by American Cyanamid of Bridgewater, N.J., and sold under the trademark "Cyasorb UV-531." The UVS functions as a "free radical scavenger" to neutralize free radicals within the polymeric matrix. Conventional UV stabilizers include bis-(tetramethyl piperidinyl sebacate), produced by Ciba-Geigy Corporation of Hawthorne, N.Y., and sold under the trademark "Tinuvin 770," and hindered amine light stabilizers (also known as "HALS"), such as those described in U.S. Pat. No. 5,447,576 to Willis. The final component, an antioxidant, is used to inhibit thermal oxidation of the polymer during thermal processing, and to minimize the concentration of free radical precursors, hydroperoxides and peroxides. In commercial formulated EVA materials, the antioxidant is tris-(mono-nonylphenyl)phosphite, produced by Uniroyal Chemical Corporation of Middlebury, Conn., and sold under the trademark "Naugard P. "
Despite the use of these conventional stabilizing additives, most EVA materials discolor relatively quickly to a yellow or brown color which, of course, interferes with full spectrum solar admission to the encapsulated PV device. Since the encapsulated device as well as its polymeric encapsulant are usually sandwiched within cooperating structural-assembly components, replacing the discolored encapsulant is not practical. As a result, and after only a relatively short period of time, solar transmission to the encapsulated PV device is permanently reduced and energy-collection efficiency drops dramatically. For example, browning of the EVA encapsulant reportedly reduces the annual energy output of the Carrisa, Calif., power plant by about 30%. See Gay, C. F., and E. Berman, Chemtech (March 1990), pp. 182-186. According to Rosenthal and Lane, the average module power output is approximately 35.9% below that of the initial performance rating, with the reduction in performance being highly variable between individual modules. See Rosenthal, A. L., and C. G. Lane, Proc. PV Module Reliability Workshop, Oct. 25-26, 1990, Lakewood, Colo., SERI/CP-4079, pp. 217-229. Moreover, mismatching between contiguous modules, due to severe EVA browning and nonuniform electrical degradation, reportedly reduces the power output by an additional 11.1%. See Kusianovich, V. J., ibid., pp. 241-245.
The foregoing field observations resulted in the so-called "EVA browning crisis" in 1990, which precipitated concerns about the future of PV power generation. Recent reports indicate that EVA browning reduces the power output of c-Si PV modules at the Ben-Gurion National Solar Energy Center in Sede Boqer, Israel, by about 1% per year. See Czanderna, A. W., and F. J. Pern, Solar Energy Materials and Solar Cells (1996) 43: 101-183; Berman, D., and D. Faiman, Proc. PV Module Reliability Workshop, Oct. 25-26, 1990, Lakewood, Colo., NREL/CP-411-7414, pp. 289-312. In addition to causing transmittance loss and mismatching between adjacent solar cells, researchers report a number of other effects of EVA degradation on PV modules. Such additional effects of EVA degradation include blistering of Tedlar backing foils due to the accumulation of gases from EVA photothermal degradation, delamination of EVA and solar cells from glass superstrates, and oxidation and corrosion of tinned copper ribbons (tabs) under browned EVA films. Such oxidation and corrosion of the metallic components can increase contact resistance, reduce the current collection efficiency, cause mismatching, and further decrease the power output of the module.
The current commercial EVA material, sold under the trademark "Elvax 150" by DuPont of Wilmington, Del., softens to a viscous melt at temperatures above 70.degree. C. Thus, to provide a thermostable material at typical operating temperatures, the polymer must be chemically cross-linked by processing with a suitable curing agent. Curing agents used specifically for EVA include 2,5-dimethyl-2,5-di-t-butylperoxy-hexane and O,O-t-butyl-O-(2-ethylhexyl) mono-peroxy-carbonate, manufactured by Elf Atochem, Buffalo, N.Y., and sold under the trademarks "Lupersol 101" and "Lupersol TBEC," respectively.
The degree of chemical cross-linking of the cured encapsulant, generally referred to as "gel content," is expressed as the fraction (percentage) of the polymer that cannot be extracted using a suitable solvent, such as toluene or tetrahydrofuran. To provide sufficient mechanical strength to support the solar cells in a PV module, the gel content of the encapsulant must be at least 70%. The requisite gel content is typically achieved during the lamination and curing cycles of module processing. However, the degree of cross-linking in cured EVA gradually increases over time as a result of photo-oxidative degradation. See, e.g., Liang, R. H., et al., "Photothermal degradation of ethylene-vinyl acetate copolymer, Polym. Sci. Technol. (1983) 20: 267-278; and Pern, F. J., and A. W. Czandema, "Characterization of ethylene vinyl acetate (EVA) encapsulant: Effects of thermal processing and weathering degradation on its discoloration, Sol. Energy Mater. Sol. Cells (1992) 25: 3-23. As discussed above, excessive cross-linking (ie., a gel content greater than 90%) can compromise the mechanical properties of the encapsulant, such as elasticity and tensile strength.
Thus, although a fairly high initial gel content is required for thermal stability, increases in the cross-linking density can provide an additional indication of the extent of polymer degradation.
Conventional cured encapsulants generally contain high concentrations of UV-excitable chromophores. The uncured commercial EVA material, "Elvax-150," comprises short .alpha.,.beta.-unsaturated carbonyl groups, which act as photosensitizers for the photodegradation of the EVA encapsulant. During thermal processing (curing), these .alpha.,.beta.-unsaturated carbonyl groups react to produce new chromophores. See Pern, F. J., and A. W. Czanderna, supra. Although most conventional EVA formulations also contain "Cyasorb UV-531," the UVA absorbs relatively short-wavelength radiant energy, i.e., below 360 nm. Thus, because "Cyasorb UV-531" is ineffective at absorbing long-wavelength UV light (i.e., greater than 360 nm), it does not provide complete protection against UV-activation of curing-produced chromophores. The foregoing discovery by the present inventor explains why photochemical degradation remains a problem for conventional cured encapsulants, despite the presence of "Cyasorb UV-531."
Recent reports show that the curing process produces additional UV-excitable chromophores, the concentration of which depends upon the particular curing agent and curing conditions (temperature, time and pressure). Pern and Glick report that, because of their relative curing times, the "slow-cure" agent ("Lupersol 101") produces significantly more chromophores than the "fast-cure" agent ("Lupersol TBEC"). See Pern, F. J. and S. H. Glick, "Fluorescence analysis as a diagnostic tool for polymer encapsulation process and degradation," In: 12th NREL Photovoltaics Program Review, AIP Conf. Proc. (Am. Inst. Physics, Woodbury, N.Y., 1994), 306: 573-585. [Cf. conventional lamination procedures generally require about 8-10 minutes at 120.degree. C., whereas the typical curing time is about 40-50 minutes at 145.degree. C. using "Lupersol 101," and about 8-10 minutes at 145.degree. C. using "Lupersol TBEC." See, e.g., E. F. Cuddihy, et al., Flat-Plate Solar Array Project Final Report, Vol. VII. Module Encapsulation, JPL Publn. (1986) 86-11.] "Lupersol TBEC" is essentially depleted within about 20 minutes of thermal processing at 145.degree. C., due to its low activation energy and short half-life. In contrast, approximately 42% of "Lupersol 101" remains after 45 minutes of curing. This high residual "Lupersol" content is significant, since this peroxide is believed to be the primary oxidizing reagent responsible for the generation of chromophores. See Pern and Glick, supra. Finally, in addition to increasing the concentration of curing-generated chromophores, longer curing times also provide the opportunity for additional deacetylation reactions, which produce acetic acid, a known EVA discoloring agent. See Czanderna and Pern, supra.
To summarize, it is desirable to produce a cured encapsulant having minimal curing peroxide residue, since peroxides promote free radical formation. It is also desirable to minimize the concentration of curing-generated chromophores, since these chromophores can enhance the EVA discoloration reactions and rate. Thus, the "fast-cure" agent, "Lupersol TBEC," is generally preferred over the "slow-cure" agent, "Lupersol 101," since the former curing agent produces lower concentrations of curing peroxide residue and UV-excitable chromophores.
Although "Lupersol TBEC" produces relatively low concentrations of curing-generated chromophores and residual curing peroxide as compared to "Lupersol 101," and thus is the preferred curing agent for EVA encapsulating materials, the use of "Lupersol TBEC" is problematic for other reasons. "Lupersol" peroxides decompose during curing and produce gaseous organic products. When produced in sufficient amounts, these gaseous products cause voids or bubbles to form in the encapsulant, which reduce the optical coupling to, and hence the conversion efficiency of, the photovoltaic device. Voids can also cause delamination of the EVA and solar cells from glass superstrates. Unfortunately, the fast-curing "Lupersol TBEC" tends to produce relatively high amounts of gaseous decomposition products. Therefore, care must be taken to control the lamination and curing conditions to minimize the potential for bubbling.
Another problem associated with conventional formulations of encapsulating materials for PV devices is the generally poor stability of the antioxidant compound against moisture and thermal decomposition. Existing commercial EVA formulations include the antioxidant "Naugard P," which Ad tends to hydrolyze in the presence of moisture (water). In addition, "Naugard P" tends to oxidize during the cure cycle, particularly when using slow-cure agents such as "Lupersol 101." Both of these factors reduce the effectiveness of "Naugard P" in protecting EVA against thermal oxidation.
A need therefore exists for an improved encapsulating material for photovoltaic devices. This improved encapsulating material should feature a variety of desirable chemical and physical properties. Specifically, the encapsulating material should provide improved photothermal and photochemical stability, a minimal concentration of curing-generated chromophores, a relatively fast curing rate, the desired degree of cross-linking, minimal gas formation (bubbling), and improved stability of the antioxidant against moisture and thermal decomposition.